\documentclass[12pt]{article}  
 \textheight15cm \textwidth11cm \parindent0mm \parskip 1ex plus2ex minus0.8ex
\begin{document}
\newcommand{\lt}{this long text}
It is annoying to have to type  \lt\ so often. \newline 
It is annoying to have to type  \lt\ so often. \newline 
It is annoying to have to type  \lt\ so often. \newline 
\renewcommand{\lt}{this even longer text}
It is annoying to have to type  \lt\ so often. \newline 

\newcommand{\emcorr}[1]{{\em #1\/}}
This text will be printed using a \emcorr{emphatic} type.

\newcommand{\Gi}[1]{\mbox{$\Gamma_{#1}$}} % the mbox is important because of math mode!
%\newcommand{\Gi}[1]{\ensuremath{\Gamma_{#1}}} % alternative to mbox in LaTeX2e
\Gi{1} and \Gi{2} being numbers with $\Gi{i}\geq 0$.

\newenvironment{emphit}{\begin{itemize}\em}%  <- this percent character is important, see above
                       {\end{itemize}}
Now something special:
\begin{emphit}
 \item This nice environment produces emphasized items.
 \item That is very nice, isn't it?
\end{emphit}

In LaTeX2e only:
\newcommand{\example}[2][YYY]{\par Mandatory arg: #2; Optional arg: #1.}
\example{BBB}
\example[XXX]{AAA}
\end{document}
